It is sometimes desirable to know the specific level of a liquid in a tank or container. Many different techniques have been developed to accomplish this objective. Some are mechanical, as by determining the level of the liquid from a wetted dip stick; some are optical; and still others are electrical.
U.S. Pat. No. 3,898,638 discloses a differential temperature sensor in which a heated electrode 16 is located at different distances from two sensing probes. If fluid flows through the conduit in which the probes are mounted, the two sensors do not sense an appreciable temperature differential therebetween. On the other hand, if the flow through the conduit stops, then the sensing probes will discern a differential change in the electrical resistivity of the sensing probes.
U.S. Pat. No. 4,449,403 discloses a guide tube-inserted liquid level sensor. This device as a chain of series-connected resistance temperature devices ("RTDs") to measure the temperature gradient within a guide tube. As shown in FIG. 2 of this reference, there are two RTDs, indicated at 29 and 38. One RTD is heated by a proximate heater 30. The sensors are wetted. Boiling and void conditions are sensed by the difference in resistance of the RTDs.
U.S. Pat. No. 4,532,799 discloses a liquid level sensor having a probe that is adapted to be inserted into a container having a liquid therein. A resistance wire is helically wound on a core within a heating element. More heat is carried away by the vapor than by the liquid. Hence, the electrical resistance of the entire length of wire (i.e., the resistance of the portion in the vapor and the resistance of the portion in the liquid) will provide an analog signal indicative of the change in liquid level. However, upon information and belief, it is critically important to know the temperature of the air and the temperature of the liquid in order to determine the particular level of liquid. In other words, the device shown in the '799 patent appears to be useful in showing a change of liquid level, but not indicating the absolute analog value of the liquid level itself.
Finally, U.S. Pat. No. 5,210,769 discloses a liquid level measuring system for a vehicle. Basically, the fluid levels are determined as a function of the electrical resistance of a positive temperature coefficient conductor, part of which is wetted and part of which is not. This patent discloses that such sensors may be used to determine the level of engine oil, coolant, windshield washer fluid, transmission fluid, and fluid in a differential.
Accordingly, it would be generally desirable to provide an improved resistance-type sensor that can be used to determine the absolute level, as opposed to a change in level, of a liquid within a container on an analog basis.